What is the native resolution of the Sharp Display? For optimal quality you should set your mac to output at the Native resolution of the Sharp LCD (this is true of any LCD). See System Preferences> Display Preferences to change screen resolution. Also make sure your sharp's built in screen settings (not the ones you adjust from the Macs display preferences) are set properly. For example, through the remote you can choose settings for "PC", "Movie", "TV", etc. I usually find that the Sharps "PC" default settings are very bad... too much contrast, bad color reproduction. so I have to manually tweak them (also done through the remote). Best of luck. I am confident you can get an excellent image out of your Sharp.
Regarding creating multiple user accounts, I highly recommend you start a separate post on that topic since it is so unrelated to your sharp lcd issue. I am confident you will get lots of responses to a post about that. Try to provide as much detail as you can about what you want, and why you think you need multiple user accounts (for example, "I want to share music, but I don't want my kids mucking about with my files" )... and you might want to consider posting that thread in the OS X topic area (here:
http://discussions.apple.com/forum.jspa?forumID=752)
But for the record, I really try to avoid setting up multiple user accounts if possible because it is a pain to share iTunes music and movies (though its possible). I think the only reason to set up multiple users is when you have kids and you want to prevent them from messing up your own settings and files. If it is two adults sharing the same computer, I think it is best to just compromise on how you configure the doc, and to agree to save files in different folders inside documents. work arounds for email can also be done from one acount. best of luck